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A Review of Digital Techniques for Modeling Vacuum-Tube Guitar

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A Review of Digital Techniques for Modeling Vacuum-Tube Guitar Jyri Pakarinen∗ and David T. Yeh† A Review of Digital ∗Department of Signal Processing and Acoustics Techniques for Modeling Helsinki University of Technology P.O. Box 3000 FI-02015 TKK Finland Vacuum-Tube Guitar [email protected] †Center for Computer Research Amplifiers in Music and Acoustics Department of Music Stanford University Stanford, California 94305-8180 USA [email protected] Although semiconductor technologies have dis- as software plug-ins so that the musician can placed vacuum-tube devices in nearly all fields of connect the guitar directly to the computer’s sound electronics, vacuum tubes are still widely used in card, record the input tracks, add effects and/or professional guitar amplifiers. A major reason for virtual instruments, and then compile the song as this is that electric-guitar amplifiers are typically a CD or upload it to the Internet. This is especially overdriven, that is, operated in such a way that the useful for home studios and small ad hoc recording output saturates. Vacuum tubes distort the signal in sessions, because it eliminates several tedious a different manner compared to solid-state electron- tasks of acoustic recording, such as setting up the ics, and human listeners tend to prefer this. This amplifier and recording equipment, selecting a might be because the distinctive tone of tube am- microphone position, finding a recording room, etc. plifiers was popularized in the 1950s and 1960s by This article attempts to summarize real-time early rock and roll bands, so musicians and listeners digital techniques for modeling guitar tube ampli- have become accustomed to tube distortion. Some fiers. Although a brief overview was presented in studies on the perceptual aspects of vacuum-tube Pakarinen (2008), to the authors’ knowledge, there and solid-state distortion have been published (e.g., are no previous works that attempt a comprehensive Hamm 1973; Bussey and Haigler 1981; Santo 1994). survey of the topic. Because this topic is relatively Despite their acclaimed tone, vacuum-tube new and commercially active, most of the reference amplifiers have certain shortcomings: large size and material can be found in patents rather than aca- weight, poor durability, high power consumption, demic publications. Judging from the large number high price, and often poor availability of spare parts. of amateur musicians and home-studio owners, as Thus, it is not surprising that many attempts have well as the huge number of discussion threads on been made to emulate guitar tube amplifiers using Internet forums, this topic is potentially interesting smaller and cheaper solid-state analog circuits (e.g., for a wide spectrum of readers. Thus, a conscious Todokoro 1976; Sondermeyer 1984). The next step choice has been made to try to survey the modeling in the evolution of tube-amplifier emulation has techniques at an abstracted level, without delving been to simulate the amplifiers using computers into the underlying mathematics or electric circuit and digital signal processors (DSP). analysis. A primary advantage of digital emulation is that This review is organized into four sections. We the same hardware can be used for modeling many first describe the sources of the nonlinearities in different tube amplifiers and effects. When a new guitar amplifier circuits. Then, we review published model is to be added, new parameter values or methods for modeling the linear stages of guitar program code are simply uploaded to the device. amplifiers. The heart of this survey is the review Furthermore, amplifier models can be implemented of methods for nonlinear modeling. Finally we Computer Music Journal, 33:2, pp. 85–100, Summer 2009 c 2009 Massachusetts Institute of Technology. Pakarinen and Yeh 85 Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/comj.2009.33.2.85 by guest on 26 September 2021 Figure 1. Physical construction (a) and electrical representation (b) of a triode tube. (Figure (a) is adapted from en.wikipedia.org/wiki/Vacuum tube.) mention various other guitar-amplifier related technologies and present conclusions. Vacuum-Tube Amplifiers The purpose of this section is to present an overview of the operation of vacuum-tube amplifiers and to illustrate the complex nature of their important nonlinearities. An overview of vacuum tubes used in audio applications can be found in Barbour (1998), and a detailed tutorial on classic vacuum- tube circuits is provided in Langford-Smith (1954). The physical principles governing the operation of vacuum tubes are reviewed in Spangenberger (1948). Excellent Internet articles discussing the design of guitar tube amplifiers can be found online (e.g., at www.aikenamps.com and www.ax84.com). A typical guitar tube amplifier consists of a commonly used for signal rectification. Three- preamplifier, a tone-control circuit (i.e., tone stack), terminal devices are known as triodes and are a power amplifier, and a transformer that couples primarily used in preamplifier circuits. Four- and to the loudspeaker load. The preamplifier magnifies five-terminal devices (tetrodes and pentodes, re- the relatively weak signal from the magnetic spectively) are used mainly for power amplification guitar pickups and provides buffering so that the purposes to drive a loudspeaker, for example. pickup response is not altered by the amplifier The operation of vacuum tubes is analogous to circuitry. The preamplifier is usually realized with water flow on a slope. First, the electrode termed triode tubes. The tone stack provides a typical V- the cathode is heated, and the process known as shaped equalization for compensating the pickup’s thermionic emission acts like a pump that forms resonance at mid-frequencies, and it gives the a pool of electrons at the top of a hill. A second user additional tonal control. The power amplifier terminal called the plate (or anode) is at the bottom boosts the signal so that it is powerful enough of a slope. Electrons will flow from the cathode to to drive a loudspeaker. In the so-called all-tube the plate depending upon the relative height of the guitar amplifiers, both the pre- and power-amplifier plate, which is controlled by the voltage applied circuits use tubes instead of transistors in amplifying to it. Note that because a pump is at the cathode, the signal. Typically, these amplification circuits electrodes can never flow backward from the plate contain one or more tube stages, namely, circuit to the cathode even though the plate may be raised blocks that consist of a tube connected to resistive uphill of the cathode. This describes the rectification and capacitive (RC) components. behavior of a diode tube. The triode, illustrated in Figure 1, introduces a third terminal called the grid between the two Vacuum Tubes terminals. With the plate downhill of the cathode, the grid is like a raised barrier in the slope that Vacuum tubes, or thermionic valves, were invented limits the flow of electrons from the cathode to in the early 1900s for amplifying low-level volt- the plate. If this barrier controlled by the grid is age signals. Structurally, they consist of two or high enough, it stops the electron flow completely. more electrodes in a vacuum enclosed in a glass This water-flow analogy motivates the British term or metal shell. A two-terminal device is a diode, referring to vacuum tubes as “valves.” 86 Computer Music Journal Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/comj.2009.33.2.85 by guest on 26 September 2021 Nonlinear Amplification of the input signal, the grid current Igk charges the capacitor and dynamically varies the bias point of The plate-to-cathode current is a nonlinear function the tube, leading to dynamically varying transient of both the grid-to-cathode and plate-to-cathode distortion characteristics. voltages: Ipk = f (Vgk,Vpk). Note that a change in The cathode bypass capacitor retains memory of voltage on the grid causes a change in current the tube bias and responds slowly to rapid changes in flow between the cathode and plate. Amplification signal amplitude, causing signal history–dependent occurs when the change in current is converted to changes in distortion characteristics. Furthermore, a change in voltage by a large-valued load resistor. there exist parasitic capacitances in the tube itself Although amplification is nominally linear around owing to the close proximity of its electrodes. The a central operating voltage known as the bias,at dominant effect, Miller capacitance, is a low-pass extreme signal levels, the amplified output will filter resulting from the amplified capacitance saturate. When the grid-to-cathode voltage Vgk is between plate and grid; this is discussed more very small, current flow cuts off sharply. Very large thoroughly in Aiken (1999a). Vgk causes the plate voltage to approach that of the cathode again, limiting the current and resulting in a nonlinearly saturating characteristic. To find the Amplifier Power Stage full nonlinear transfer characteristic from input to output requires the solution of a nonlinear system The power amplifier can use either a single-ended of implicit equations, because in a typical amplifier or push–pull topology. In the single-ended topology, circuit, Vpk depends on Ipk and vice versa. the signal is amplified in a single vacuum tube. This In guitar-amplifier circuits, the operating point tube conducts plate-to-cathode current during the (bias), defined in terms of current through the tube whole signal cycle (Class A biasing). Parallel tube device, is often set by a resistor connecting the stages can also be added if more output power is cathode terminal to ground. The resistor introduces required. feedback into the circuit, and its value influences The push–pull topology, perhaps more commonly the shape of the input-output curve and determines used, consists of two identical sets of output tubes the offset about which the signal varies.
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